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By David Keith | April 30, 2026

In late 2025, Stardust, a for-profit startup, announced it had raised $60 million in venture capital funding to develop a proprietary system for solar geoengineering. The company’s mission is to commercialize sunlight reflection methods (SRM) by deploying specially engineered, “chemically inert” (i.e., safe) particles into the stratosphere via custom aircraft. This is a system that Stardust claims could be operational by the start of the next decade.

I want to see a technically detailed plan for the early deployment of sunlight reflection using sulfur in the stratosphere. Such a plan should extend from materials handling and dispersal from aircraft, through monitoring hardware, data assimilation, and the feedback controls needed to ensure that deployment achieves a goal such as ramping up cooling at a rate of 0.1 C per decade—roughly halving current warming—while maintaining hemispheric balance.

I want this plan to be detailed and public. Such a plan would inform decisions about governance and deployment. A critical review of SRM requires such a plan.

The SRM research community is doing an inadequate job of producing such a plan. So, there is an unmet need.

Stardust has a strong technical team with systems engineering expertise that could address this need. Yet I don’t expect Stardust to meet this need, first because as an opaque, for-profit company, it has the wrong structure; and second, it’s focused on the wrong problem.

Daniele Visioni and I argued that Stardust has the wrong structure in our 2025 op-ed in MIT Technology Review; proprietary technology and profit motives are directly at odds with the transparency needed to legitimize SRM research. As we said, “trust will be the most important single ingredient in making these decisions. And trust is the one product for-profit innovation does not naturally manufacture.” That is why I want to see a public not-for-profit doing this work instead.

Here I argue that Stardust is focused on the wrong problem. Stardust seems focused on finding an alternative aerosol that is better than sulfates. Yet the direct risks of sulfur—as opposed to other SRM aerosols—are quantitatively small and well understood, whereas the solutions Stardust advances are all-but-certain to introduce new unknown unknowns, which might entail potentially large, unanticipated risks.

As I see it, the main attraction of Stardust rests on two assumptions¹:

1. Sulfur is a risky way to do SRM, so there’s a big benefit in finding particles that are less dangerous than sulfur.
2. If an SAI aerosol was made from something non-toxic it would be safer than aerosol made from sulfate.

Let’s start with assumption #1. Suppose one could completely remove the direct health impacts from the sulfate aerosols injected into the stratosphere? Suppose there were a magic aerosol that causes zero environmental or health impacts as it makes its way down to the ground and into lungs and ecosystems. How much would this change the risk of SRM?

Not much.

While SRM will impact human health though changes in air quality, sulfuric acid’s direct contribution to that harm is surprisingly small.

Seb Eastham’s 2018 paper showed that the direct impacts of sulfate SRM on air quality are small compared to the indirect effects of the SRM’s cooling and other climate change on air quality.

The amount of aerosols in the air we breathe is most commonly measured as PM_2.5, the total mass in micro-grams of all aerosol particles smaller than 2.5 µm in each cubic meter of air.

The epidemiological evidence that aerosol particles are unhealthy is very strong. Very roughly, this data shows that if you are exposed to an extra ~30 µg/m³ of PM2.5 your life expectancy decreases by about a year.

We expect that aerosols added to the stratosphere will make their way to the surface atmosphere and add to PM_2.5 causing harm. Seb Eastham aimed to quantify this impact of SRM. To his surprise, he found that while stratospheric aerosols descending to the surface did contribute to PM_2.5, that contribution was small compared to the increase in PM2.5 caused by cooling. Why did cooling make more PM_2.5? Mostly because it increased the amount of nitric acid in aerosols, nitric acid that mostly comes from industrial emission of NOx. (For more details see section 3.2 of Seb’s paper.)

Now, a new study from Daniele Visioni’s group using more modern methods has confirmed that result.

Assumption #1 is therefore false. It would be great to eliminate the direct air quality impacts of sulfate SAI aerosol, but these impacts are quantitatively small compared to the indirect effects of SRM on air quality.

Context matters: air quality impacts do not seem to be seen as a leading concern about SRM. (See my list of concerns here). Moreover, the combined impacts of sulfate SRM on human health counting aerosols (PM_2.5), ground level ozone, and increased UV caused by damage to stratospheric ozone, are at least 10X smaller than the health benefits of SRM in the form of reduced deaths from heat. See Tony Harding’s paper in PNAS, and our commentary.

Onto assumption #2. If an SAI aerosol was made from something organic and “non-toxic,” it would be safer than an aerosol made from sulfate. This statement seems so obviously true as to be inarguable. Drink a glass of sulfuric acid and you die; eat a tablespoon of flour and you’re fine. Of course, an SAI aerosol made from organic, edible food-safe material would be safer than an aerosol made of sulfuric acid, right?

Maybe not.

Materials that are safe to eat or drink can sometimes be dangerous if inhaled. Wheat flour is safe to eat (unless one is intolerant). Yet inhaling half micron wheat-flour aerosol particles is dangerous.

The health hazards of aerosol particles cannot be deduced from the biocompatibility or toxicity of the materials out of which they are made.

While there are well established protocols for testing the safety of materials ingested in food or drink, there are no similar protocols for testing aerosol safety, particularly not chronic exposure to low concentrations of aerosols.

In many environments, sulfate is a good fraction of total mass of PM_2.5, so regulations have pragmatically focused on sulfates.

But that does not mean that it’s the sulfuric acid in the aerosols that is causing harm. Despite decades of studying the health effects of PM_2.5, scientists are still uncertain about what specific components of particulate matter are causing the harm.

Evidence suggests sulfate itself may not be the root cause. Mice exposed to pure sulfate aerosols in the lab suffer comparatively low health impacts. Here’s a recent paper.

A leading hypothesis for why PM_2.5 aerosols are dangerous is the so-called “hitchhiker hypothesis,” which is the idea that these aerosols pick up tiny amounts of organics or metals and transport them into the lungs. Once there, the hitchhiking organics or metals cause the harm, not sulfate itself².

So, it’s plausible that some organic, food-safe aerosol could be more dangerous than sulfate if the main driver of toxicity is the hitchhiking organics or metals and not sulfate itself.

Moreover, a Stardust-engineered aerosol injected into the stratosphere will undergo complex chemical reactions as it descends though the atmosphere and reaches someone’s lungs. So, a test of toxicity of the pure stardust aerosol could not accurately predict the impact in the real world just as tests of the toxicity of pure sulfate aerosol do not correctly predict the health impacts of PM_2.5 aerosols.

Assumption #2 is therefore unknown. An SAI aerosol made from something non-toxic might be safer or less safe than an aerosol made from sulfate.

What we can say for sure is that a novel aerosol would have more uncertain health impacts than sulfate. There would be more basis for concern about unknown unknowns.

Sulfur aerosols are not safe. We know they cause mortality and morbidity, but we can estimate the size of their impacts with some confidence because of many thousands of scientific studies over the last century. This means there is little basis for concern about unknown unknowns with sulfate.

Summary

Stardust has the wrong structure because trust is the one product for-profit innovation does not naturally manufacture.

Stardust is focused on the wrong problem because (a) the direct health impacts of SAI sulfate can be assessed with some confidence using the immense scientific literature on their health impacts; and (b) assessments using this literature find that the direct health impacts of SAI sulfate are a small contributor to the overall risks of SAI; and finally, (c) the health and environmental risks of a novel particle would be more uncertain than the risks of sulfates.

I am no naïve booster of sulfates—I published some of the early papers on non-sulfate aerosols³. In addition to reducing health impacts, designer aerosols offer the chance to reduce three important side effects of sulfate SRM: heating of the lower stratosphere, damage to the ozone layer, and alteration of the visual appearance of the sky. It’s worth researching better methods of SRM.

But these three impacts only grow large when a large amount of sulfur is used, as would be needed to cool Earth more than about 0.5 or 1 degree Celsius. If SRM is gradually ramped up (in my view the only sensible way to use it) it will take a long time—perhaps half a century—until SRM is cooling Earth enough for these side effects to be important. Thus, it will be a long time before there is a sound reason to use designer aerosols.

In the long run, some new aerosols will be demonstrated that are better than sulfate. But it is very hard to argue the humanity should start with anything other than sulfate, because of the depth of our knowledge about its hazards and the inherent uncertainty in any designer particle.

I am convinced there is an urgent need for end-to-end systems engineering for SRM. While I disagree with what Stardust is doing, my conversation with the Stardust founders left me with a strong sense that they have good intentions. I believe their work is motivated by a shared desire to do this systems engineering for SRM. I wonder if they over-focused on the ‘ideal particle’ concept because it’s easier to sell novel tech to investors than it would be to raise funds developing a sulfate aerosol system.

In any case, the rise of Stardust has convinced me of the need for a public non-profit entity that can do this systems engineering for SRM using sulfate aerosols.

Acknowledgements: I thank Ben Peltz and Dakota Gruener for editorial suggestions.

Notes

¹ On April 2, 2026 Stardust released a “A proposal for the safety and controllability requirements that SRM systems should meet” on arXiv.

² See, https://hsph.harvard.edu/news/metals-and-sulfate-in-air-pollution-mixture-may-contribute-most-to-asthma-hospitalizations/, or https://documents1.worldbank.org/curated/en/810141630705865331/pdf/Are-All-Air-Pollution-Particles-Equal-How-Constituents-and-Sources-of-Fine-Air-Pollution-Particles-PM-2-5-Affect-Health.pdf

³ https://davidkeith.earth/publication/photophoretic-levitation-of-engineered-aerosols-for-geoengineering/, https://davidkeith.earth/publication/solar-geoengineering-using-solid-aerosol-in-the-stratosphere/, https://davidkeith.earth/publication/stratospheric-solar-geoengineering-without-ozone-loss/.

By David Keith | September 21, 2025

Supplementary comments on “A responsible way to cool the planet” with Zeke Hausfather, an opinion essay in the NYT published 21 September 2025.

My guess is that a fair-minded understanding of the risks and benefits of deploying a small amount (example below) of sunlight reflection would yield surprisingly broad support for limited deployment.

Yet, any talk of limited deployment comes with one giant caveat: the fear that once started, it would be politically hard to stop. If there’s a big problem, it’s easy to stop. But if it starts and goes roughly to plan, then there will be a strong temptation to keep doing more, while weakening efforts to cut emissions or remove carbon.

How to set up political structures that avoid over deployment? There are clever proposals for voting rules that tie the right to vote in decisions about SRM to successful emissions cuts, but there are no easy answers.

Zeke and I propose a simple rule: limit use of SRM to maintaining Earth’s reflectivity against the decrease in reflectivity that will continue as pollution is cleaned up. This rule has two useful consequences. First, it ties use of SRM to clean up of pollution, and since this pollution is mostly from burning fossil fuels, this is roughly equivalent to tying use of SRM to cuts in fossil fuels; and second, it provides a non-arbitrary fixed upper limit on how much SRM can be deployed.

One can imagine this rule is simple enough to be useful in international negotiation over sunlight reflection.

Any such limit can obviously be overridden. It’s impossible to bind the hands of the future, and it would be unwise to do so, even if it were possible.

Notes:

Here’s what a small amount could look like: injecting sulfur into the stratosphere using high-flying aircraft. Starting in the early 2030s and gradually increasing over time, the injection could, after about 50 years, reflect enough sunlight to cool Earth by 0.5 C. That would lower projected 2080 temperatures from about 2.5 C above pre-industrial levels without SRM to around 2.0 C, while balancing the cooling across both hemispheres. This a bit larger than current cooling now caused by pollution, which is more like 0.4 C, but it’s similar magnitude. The human and environmental benefits of reducing temperatures by half a degree are enormous as demonstrated by the IPCC 1.5 report.

A strong temptation to keep doing more. This is geoengineering’s moral hazard, perhaps better called “mitigation deterrence”. It can arise from political exploitation or collective addition, as I describe here: https://davidkeith.earth/publication/toward-constructive-disagreement-about-geoengineering-a-shared-taxonomy-of-concerns-may-help/ . It can arise from political exploitation or collective addition as I describe here: https://davidkeith.earth/publication/toward-constructive-disagreement-about-geoengineering-a-shared-taxonomy-of-concerns-may-help/

Here are my previous NYT op-eds:

The New York Times: What’s the Least Bad Way to Cool the Planet?

The New York Times: Blocking the Sky to Save the Earth

By: David Keith and Anthony Harding (Georgia Tech)

Climate change has risks—and those risks are only increasing. Many of these might be reduced by solar geoengineering. Solar geoengineering also has risks, and it requires rigorous, transparent research before it is deployed. One major question though is how does the reduction in risks from solar geoengineering compare to the additional risks its use entails? Or equivalently, how big are the benefits of solar geoengineering compared to its harms?

Our paper, Impact of solar geoengineering on temperature-attributable mortality, is a first effort to provide a quantitative risk-risk comparison for any solar geoengineering method. It answers the call of an emerging consensus that if we’re going to avoid the worst effects of climate change, we need to evaluate every potential solution—including solar geoengineering. Importantly, highly credible scientific organizations like the National Academies of Sciences, Engineering, and Medicine (NASEM), and a growing chorus of experts, are among those who recognize solar engineering deserves exploration.

In our new paper, we quantify and compare what we consider to be a few of the largest physical risks for sulfate aerosol injection—mortality risk from temperature, air pollution, and ozone loss.

Injecting sulfate aerosol into the stratosphere will cool the planet, reducing mortality from heat, one of the leading risks of climate change. Sulfate aerosol air pollution is a leading cause of environmental mortality worldwide, so it is one of the most obvious risks of sulfate aerosol geoengineering. Sulfate aerosols in the stratosphere can also damage the ozone layer, causing an increase in mortality from skin cancers.

Comparing these three risks, we find that the reduction in mortality from cooling—a benefit—is roughly ten times larger than the increase in mortality from air pollution and ozone loss—a harm. Like any statement about solar geoengineering this result depends on the scenario we evaluated along with a host of other assumptions.

Our view is that quantitative analysis of the expected benefits and costs of a possible policy action is crucial input to sensible debate about public policy. This perspective is particularly relevant to solar geoengineering given its uncertainties, risks, and distributional effects. Good benefit-cost analysis should consider structural uncertainties and consider the distribution of those impacts across different affected groups. Benefit-cost analysis should not (and does not) mechanically determine policy outcomes, but good policy analysis and debate should take benefit-cost analysis seriously. Note that our view about the importance of benefit-cost analysis is reflected by the fact that we have both taught this topic in public policy schools.

In the remainder of this essay, we offer some notes and then a set of answers to questions we imagine readers might ask.

Some notes:

• Our paper is a collaboration between us and Princeton collaborators Gabe Vecchi and Wenchang Yang.
• Our paper relies on a state-of-the-art method for estimating the impact of warming on local mortality led by my UChicago colleague Michael Greenstone. The estimate of added mortality due to the addition air pollution and ozone loss comes from Seb Eastham’s paper.
• Our paper links to two prior papers. Tony led a paper on the impact of solar geoengineering on income inequality that used related econometric methods. David was part of prior collaboration with Gabe Vecchi which produced an important estimate of solar geoengineering’s potential to reduce regional climate hazards.
• Many groups have called for risk-risk analysis of solar geoengineering including the National Academy, NASEM 2021, The Carnegie Climate Governance Initiative, C2G and the call-for-balance letter with Peter Singer, James Hansen, and Bjorn Stevens, as signatories, see paper.
• The air pollution mortality estimate by Eastham combines the direct impacts of injected aerosol that makes it to the surface with climate-mediate changes in the amount of surface ozone and particulate air pollution produced from given industrial emissions. Air pollution mortality due to particular matter increases as the climate cools, a larger increase in mortality than direct impact of the sulfate injected into the stratosphere (see Figure 2 of Eastham). But this effect depends on air pollution emissions which will likely be lower late in the century than is assumed in the Eastham paper. It may be better to compare our estimated change in temperature-attributable mortality to Eastham’s estimate of direct impacts of the descending injection mass—yielding a benefit-harm ratio of about 40:1.
• Our work examines only three risk pathways: temperature-attributable mortality, air pollution, and the impact of increased ultraviolet due to reduced ozone. It is just one step toward a broad quantitative risk–risk assessment of solar geoengineering. While not comprehensive, these are important risk pathways: temperature-attributable mortality may account for more than half of the monetized harms of climate change, and air pollution and ozone loss are among the most salient impacts of stratospheric sulfate geoengineering.

How will the positive and negative impacts of solar geoengineering be distributed geographically? Research consistently suggests that those who are expected to be most harmed by a warming world are those in poorer and hotter regions of the world. Broadly, we find the converse for solar geoengineering. Cooling by solar geoengineering reduces temperature-attributable mortality in hotter regions while it increases mortality in cooler regions (Figure 1 of paper). Global warming does the converse. This, combined with the fact that mortality impacts are greater when people are poorer, means that the benefits of solar geoengineering are concentrated in hotter and poorer regions.

Did we get our result by choosing an unrealistically positive scenario for deploying solar geoengineering? On the one hand, any statement about SRM is necessarily scenario dependent. Here’s how Parson and Keith put it:

SRM presents two fundamental policy-relevant scientific questions. How effectively could it reduce climate risks? And what additional harms or risks, of what severity, would it introduce? Answers to these questions about SRM’s effects rely partly on knowledge derived from scientific research, but they also depend on assumptions about how SRM is used, under what background conditions of greenhouse gas emissions and climate change. The three principal dimensions of choice in how SRM is used are how much global-average cooling or radiative forcing is pursued, how changes in radiative forcing are distributed around the world, and what SRM method is used.

If, for example, one deployed a massive amount of SRM in only one hemisphere the results would be bad.

On the other hand, our paper expresses many of its results as ratios either of risk-to-risk or of SRM’s effect to the effect if the same climate change were caused by removal of CO₂, and these ratios are strongly dependent on the amount of SRM. We expect, for example, that the 13:1 risk-risk ratios would be very roughly the same if one was cooling the world only 0.1 C or as much as 2 C, and roughly independent of how quickly emissions were cut or carbon was removed. But our scenario does assume a roughly hemispherically balanced uniform SAI deployment.

How sure are you about your results? The abstract says there is only a 60% chance that benefits would outweigh the harms. The 60% figure is almost completely driven by the uncertainty in the effect of temperature-related mortality. Forget SRM and just think about CO₂-driven climate change: though we don’t calculate it exactly, there is a large probability that the benefits of reduced deaths in cooler regions would be larger than the harms of increased deaths in hot regions—so our 60% figure really comes from the fact that the model we use doesn’t have high confidence that CO_2-driven climate change is bad for average mortality. Things look very different if you look at a hot or cold region alone.

What next? Do you believe this type of climate intervention should be deployed? And who should have the authority to make this kind of decision? Our role as scientists is to expand the base of knowledge. Governments must decide if, when and how to put theory into practice. We hope the main impact of this paper is to spur our colleagues to provide more and better quantitative comparisons between risks and benefits.

By David Keith | August 5, 2024

Good journalism builds stories out of facts. And good stories need characters.

The NY Times recognized the growing significance of debates about solar geoengineering and commissioned a story that humanized the topic by building it around a character, me.

David Gelles’ article is fair, but as the following examples illustrate, the imperative of storytelling may leave readers with an exaggerated impression of both my importance and the differences between my views and the views of other interviewees he quotes.

“My” solution.  Gelles’ article refers to solar geoengineering as “his [my] solution”. This is doubly wrong. I was a child when the idea of injecting sulfur into the stratosphere to slow global warming was first proposed. These ideas are not mine but are the work of many people over more than half a century. Moreover, solar geoengineering is not a solution, at best it’s a band-aid; a supplement to emissions cuts but not a substitute for them.

Moral hazard. The article says, “Opponents worry it would distract from the urgent work of transitioning away from fossil fuels”, adding that opponents cite “moral hazard” as one of the main risks of solar geoengineering. This is accurate, but in crediting this concern to ‘opponents’, Gelles suggests an “all-in” vs “all-out” dichotomy, which in turn suggest that people like me who are willing to contemplate deployment are blind to the risks, while people Gelles labels as opponents are blind to the benefits. Concern about moral hazard is, in fact, widespread within the solar geoengineering research community. I fear that fossil fuel interests or fossil-rich nations will exploit solar geoengineering by exaggerating its effectiveness and minimizing its risk to weaken controls on carbon emissions. I also believe I was the first to call this concern a moral hazard almost a quarter century ago[i].

Frank Keutsch. The article says “Dr. Keutsch is less sanguine than Dr. Keith when considering [SRM’s] potential risks” and quotes Frank as saying, “I compare stratospheric solar geoengineering with opiates,” and “They only treat the symptom and not the actual cause. You can get addicted to it if you don’t actually address the cause. In addition, like any painkiller, you’re going to have side effects. And then there are withdrawal symptoms, and that’s termination shock.”

I fully agree with Frank’s comments. I love his opiate analogy and use it occasionally. Many of us use similar analogies, including myself when I compared geoengineering to chemotherapy in 2010 congressional testimony. It’s possible that Frank is more concerned than me about geoengineering’s risks, but we talk often, and my overall impression is that while we see some things differently, we share a similar overall level of concern.

David Suzuki. The article quotes David Suzuki saying, “The whole notion of spraying sulfur compounds to reflect sunlight is arrogant and simplistic”, and “The fundamental problem is that we think we’re so smart that we don’t have to pay attention to nature’s boundaries.” A reader might assume that David Suzuki opposed research on solar geoengineering—the central question at issue today—yet in a conversation I had with David Suzuki in March 2022 as part of a documentary, Suzuki’s response to a question about research on solar geoengineering was “absolutely we need more information just to show us how ignorant we are.”

Shuchi Talati, the founder of a nonprofit organization called the Alliance for Just Deliberation on Solar Geoengineering, called the technology “a double-edged sword.” Adding that “…it can also exacerbate suffering if used in a bad way.”  Yes. Solar geoengineering could cause immense harm through deliberate misuse. To dramatize its terrible possibilities, I computed that solar geoengineering could theoretically be used to cool the earth over a century, freezing the oceans to the equator and, a quote that Michal Spector picked up for a 2012 New Yorker article.

Gelles’ follows Shuchi’s quote with a statement implying that I disagree, saying that I “countered that the risks posed by solar geoengineering are well understood, not as severe as portrayed by critics and dwarfed by the potential benefits”. I believe this statement is correct for the physical risks of deployments under specific conditions (e.g., hemispherically balanced and offsetting less than half the CO₂-driven warming). Yet I emphatically agree with Shuchi’s view that solar geoengineering could most certainly “exacerbate suffering if used in a bad way.” Caveats matter.

Summary. I am excited to see solar geoengineering getting above-the-fold coverage in the NY Times. Readers should bear in mind that story-driven reporting tends to exaggerate differences by portraying people as stylized representatives of sharply distinct points in a landscape of opinion.

No single person’s judgment should count for much. The news that should, in my view, matter to readers, is that there is rapidly growing agreement that research on this topic makes sense, as does active debate about how these technologies might be used and governed. There are, of course, strong voices against research from climate experts such as Ray Pierrehumbert, but evidence from surveys and formal consensus documents suggest that these opinions are held by a small minority[ii]. Additionally, to the extent we can gauge public opinion, there is a surprising level of support for research which is strongest in the developing world[iii].

If you want to learn more about solar geoengineering I suggest the One Atmosphere report from the UN Environment Program. For my views in my own words, have a look at my 2021 essay in the NY Times or my short book. My website has all my articles. If you want ones most relevant to general readers use this search. If you want my take on the links between nature and climate engineering, see my µ-autobiography. Or, see my Facts & values: my thoughts on talking about solar geoengineering.

Minor errors

• “Risk is negligible compared to the benefits”. If I said “negligible” then I misspoke, but I doubt I said negligible as I have been speaking about this comparison a lot and have a standard way to describe it. The comparison between the reduction in mortality from heat and the increase in mortality from air pollution is from a paper by Tony Harding. An early version is public as an RFF whitepaper; a full version is under peer review. I have been speaking about this quite a bit and I typically provide a quantitative comparison of benefits to harms and include caveats as we do in the paper. Since the deaths from air pollution would be due to deliberate introduction of sulfur, I don’t think it’s ethical to dismiss them as negligible.
• SCoPEx history. Firstly, SCoPEx was led by Frank Keutsch in the period the article refers to https://www.keutschgroup.com/scopex. I collaborated closely with Frank, but an account that leaves his name out overstates my role. Second, no test was planned over Arizona in 2018. Frank and I were developing hardware and working with balloon launch providers, but we were a long way from being able to schedule a flight. Third, the article says, “When details of that plan became public, a group of Indigenous people objected and issued a manifesto against geoengineering.” The manifesto was authored by ETC, founded in Montreal as part of an anti-geoengineering campaign. ETC did get signatures from some indigenous organizations but the article inaccurately characterizes the group that issued the manifesto.

[i] See page 276 of https://davidkeith.earth/publication/geoengineering-the-climate-history-and-prospect-2/

[ii] Dannenberg, A., Zitzelsberger, S. Climate experts’ views on geoengineering depend on their beliefs about climate change impacts. Nat. Clim. Chang. 9, 769–775 (2019); Dai, Z., Burns, E.T., Irvine, P.J. et al. Elicitation of US and Chinese expert judgments show consistent views on solar geoengineering. Humanit Soc Sci Commun 8, 18 (2021).

[iii] Sugiyama, M., Asayama, S., & Kosugi, T. (2020). The North–South Divide on Public Perceptions of Stratospheric Aerosol Geoengineering?: A Survey in Six Asia-Pacific Countries. Environmental Communication, 14(5), 641–656; Low, S., Fritz, L., Baum, C.M. et al. Public perceptions on solar geoengineering from focus groups in 22 countries. Commun Earth Environ 5, 352 (2024).

By David Keith | March 11, 2019

This post provides some color commentary as an FAQ about “Halving warming with idealized solar geoengineering moderates key climate hazards”, published 11 March 2019 in Nature Climate Change. Feel free to send me questions and I may add to the FAQ. See also Harvard press release and video.

This feels like the most important solar geoengineering (SG) study I have been lucky to be a part of. From my perspective, it’s more important and should get more attention than progress on our stratospheric experiment.

We use a high-resolution state-of-the-art model to go after a central policy-relevant question: what regions would be made worse off if solar geoengineering was combined with emissions cuts to limit climate risks? We find that no region is made worse off in any of the major climate impact indicators we examined. (It’s easy to cherry pick regions to make SG look great or terrible—we used standard regions from the IPCC SREX report.)

My hope is that the paper will dispel some of the common-but-false assumptions that solar geoengineering necessarily entails massive risks, that its impacts are highly unequal, and that it works for temperature but messes up precipitation. And I hope it demonstrates that further research needs to be done.

How does this matter for climate policy? 

There is strong evidence from multiple climate models that if solar geoengineering were implemented with reasonably uniform global coverage (e.g. uniform aerosols in the stratosphere) and if it’s used in combination with strong emissions cuts—as a complement, not a substitute—then it may offer major reductions in the climate risks that matter most to humans and ecosystems without making any region significantly worse off.

The possibility that solar geoengineering could enable deep reductions in climate risks is a strong argument for a serious global, open access research program aimed at better understanding the risks and efficacy of solar geoengineering. For more on what such a program might look like, here is my case for a responsible research program in the NAS Issues in Science & Technology, as well as an important paper from Douglas MacMartin and Ben Kravitz in PNAS.

Does this mean people who worry about the risks of solar geoengineering are wrong? Does this argue for deployment? 

Not at all. I have worried about this technology’s risks since the early 90’s. At this point research is still dominated by a small group of scientists. This means real danger of groupthink. We may simply be wrong.

What this paper illustrates is that it’s too early to leap to conclusions in either direction. This is true both for those who are convinced solar geoengineering will work, and for those who are convinced that solar geoengineering will cause droughts, or will harm the poor while benefiting the rich.

This paper, along with many previous by many authors, shows that solar geoengineering could have large and equally distributed benefits, but it doesn’t prove it. It is an idealized model. There are still huge uncertainties.

It’s clear that if misused, e.g., by deployment in only one hemisphere, solar geoengineering could have huge impacts. We need technically sophisticated efforts to quantify risks of plausible deployment of uniform and solar geoengineering that is used as a moderate supplement too emissions cuts. Until that work is done it’s too early to leap to conclusions.

Who’s behind this paper? Why does it matter? 

This paper started from a discussion with Gabe Vecchi (now Princeton, then GFDL) following a talk I gave at Princeton in 2016. Gabe decided to study solar geoengineering using GFDL’s new 25-km-resolution tropical cyclone permitting model. This is important because this model does a substantially better job simulating current precipitation extremes than typical models that have been used before on solar geoengineering. It’s also important because it is the first time that GFDL, the oldest and one of the best climate modeling centers, got involved in solar geoengineering research.

Gabe brought in Larry Horowitz (GFDL), one of the model’s developers, and Jie He (now at Georgia Tech). I meanwhile encouraged Peter Irvine, a postdoctoral fellow in my group, to take the lead in analyzing the data and writing the paper.

Gabe was collaborating with hurricane expert Kerry Emanuel (MIT), and as we began to look carefully at the hurricane responses, Gabe did not have confidence in the ocean-basin-by-basin regional response, so we invited Kerry to join the paper.

This new collaboration is relevant because solar geoengineering publications have been too dominated by a small group, and this brings significant new collaborators with deep climate science expertise to this important topic.

What about precipitation?  

This paper highlights a common misunderstanding about solar geoengineering: that a world with solar geoengineering would inevitably have less water availability. If all warming from rising CO₂ was offset by solar geoengineering, there would be less rain overall than in the current climate. This has led to concerns about droughts and monsoons. However, global warming increases rainfall so something which reverses this could reduce flood risk. When solar geoengineering is used with emissions cuts to halve warming, global-mean rainfall is more-or-less restored to its original levels. Moreover, while it seems reasonable to assume that less rain means that things are drier, in fact what matters more for ecosystems and farmers is water availability: rainfall minus evaporation. Solar geoengineering reduces rainfall, but it also reduces evaporation by reducing temperatures. So, a decrease in rainfall may be associated with an increase in water availability.

One of the ways this paper takes a step beyond current literature is by focusing on a larger set of variables that (we think) are more relevant to assessing real world climate impacts. Rather than just looking at temperature and precipitation, we looked at: annual average temperature, extreme temperature, extreme precipitation, precipitation minus evaporation as a proxy for water availability, and intensity of tropical cyclones. Note: while we do not highlight them in the paper, we also find that the simulation moderates changes in precipitation. More on this and some data on the seasonal response can be found in the supporting material.

Why did the paper adjust the solar constant rather than attempting a realistic simulation of stratospheric aerosols? 

Here’s the crucial paragraph in the paper:

We analyse the distribution of climate changes resulting from reducing the solar constant to offset roughly half the radiative forcing from doubling CO₂. A spatially uniform reflective stratospheric aerosol layer, which could be achieved by adjusting aerosol injection using feedback, would produce a similar radiative forcing to a solar constant reduction. Even with a uniform distribution, stratospheric sulphate solar geoengineering will differ from a solar constant reduction in that sulphates heat the lower stratosphere, perturb the ozone layer, and increase the ratio of diffuse to direct light. Each of these effects can be reduced by choices of alternate non-sulphate aerosol, though their side-effects are less well understood because there is no direct natural analogue. We nevertheless choose solar constant reduction as a benchmark because, given the diverse implementations of aerosols in models, solar modification allows more direct tests of inter-model differences in climate response to solar geoengineering.

Let me nerd out: In separate work, the group at NCAR, our group, and others have done work that suggests it is possible to adjust the injection of aerosols to achieve roughly uniform radiative forcing. No group has yet simulated this in a way that reasonably approximates the way that feedback from limb-sounds in situ measurements would be used in a stratospheric analysis/forecast system to adjust injection to achieve a specified optical depth profile. Moreover, no existing model can do a good job of simulating this because models with Eulerian grid boxes instantaneously mix aerosol or precursor emissions into the grid box, whereas material would form a linear plume after being dispersed from an aircraft. Local concentrations in the plume will be far higher than simulated in a Eulerian model. This will produce different SO_₂ oxidation rates and different rates of particle formation. Several research groups are now beginning to work together to address these problem.

Stepping back from technical complexity, this paper suggests what might be possible with a well-designed aerosol injection method. It also underscores one of the many reasons why research is needed—to better understand what such a method might look like, and what its risks and limitations might be.

Original post on Harvard.edu

By David Keith | June 4, 2018

My academic work is focused on solar geoengineering. I am also founder and part-time employee of Carbon Engineering, a Canadian company commercializing technology that captures carbon dioxide directly from the atmosphere.

It’s easy to confuse the two efforts, in part because of sloppy use of “geoengineering” to encompass a range of unrelated ideas, from planting trees to massive space mirrors between Earth and the Sun. Words matter. Critics sometimes exploit this confusion, implying that our work on solar geoengineering aims at profit, or takes funding from oil and gas companies to serve industry interests.

Pointed critiques aside, there is room for misunderstanding. A few months back, a cousin of mine who does environmental art asked if Carbon Engineering was commercializing my academic research on geoengineering. The answer is an emphatic, “No.” I oppose commercial work on the core technologies of solar geoengineering, yet I am very proud of the work Carbon Engineering is doing to commercialize carbon-neutral transportation fuels made from atmospheric CO_₂ and renewable power.

The remainder of this essay provides some personal reflections on the difference between direct air capture and solar geoengineering, differences that shape my views about the very different roles commercial interests play in their development. I also reflect on the conflicts of interest that arise from me working on both topics. I assume that you, my reader, have more familiarity with solar geoengineering than with Carbon Engineering’s work, so I start my explanation there.

Carbon Engineering

Carbon Engineering is a privately held company developing technology for direct air capture (DAC) of CO_₂ from the atmosphere. It was founded in 2009 in Calgary AB and is now based in Squamish BC. As of June 2018, we have just under 40 employees and have raised a cumulative total of 30m $US including both investments and government support.

Our research began as an academic effort to understand the cost of DAC by doing a bottom-up engineering cost analysis of a DAC system constructed using off-the-shelf technologies. Our work was motivated, in part, by what I suspected were over-optimistic claims that DAC might be very cheap. As we dug deeper, the effort gradually shifted from assessment to problem-solving. We began to innovate until we reached a point where it seemed the most effective way to enable this environmental technology was to create a company, so we could focus on practical research to de-risk the innovation and drive it towards commercialization.

Carbon Engineering’s primary business model is to use DAC to make carbon-neutral hydrocarbon fuels from carbon-free energy. Cheap solar power plus electrolysis can be used to make hydrogen at a price that gets more competitive every year. Carbon neutral hydrogen and DAC CO_₂ are combined using gas-to-liquids technologies to make transportation fuels such as aviation kerosene, diesel, and gasoline. We call this our AIR TO FUELS ^TM process. These fuels would be compatible with existing infrastructure but have no connection to oil and gas, and have near-zero lifecycle carbon emissions. They provide a way to use intermittent renewable power from sunny and windy locations to power transportation around the world. On a large scale, Carbon Engineering aims to make synthetic fuels at $1 per liter. Fuel from early plants would be more expensive, but in the long-run costs will come down below $1 per liter as the cost of solar and other low-carbon power decline along with the cost of electrolysis. When derived from oil, the same fuels now have production costs of about $0.6 per liter.

Our fuels are unlikely to beat oil in a head-to-head fight unless oil is penalized for its climate impact. Carbon Engineering has a strong business case today because policies that penalize CO_₂ emissions and reward ultra-low carbon transportation fuels are already in place and evolving rapidly. Examples include the various biofuel standards, California’s Low Carbon Fuel Standard (LCFS), and European fleet emission vehicle standards.

While synthetic fuels are our primary business case, Carbon Engineering is also exploring the use of atmospheric CO_₂ to make high-value products, and of markets that reward permanent removal of CO_₂ from the atmosphere through a combination of DAC and carbon sequestration technologies.

Conflicts of Interest

Ignoring larger questions about carbon removal and solar geoengineering, what about the conflict between my roles at Harvard and Carbon Engineering? Harvard allows faculty to spend up to 20% of their time on outside work, and I spend that working for Carbon Engineering. My view is that universities including Harvard are too willing to accept a professor’s involvement in companies that are tightly tied to their academic research. This problem seems most acute in biomedical research, but it also applies in cleantech. I try and keep the division sharp. I ended all my academic work on DAC soon after forming Carbon Engineering. I have no research grants on DAC and no students or research staff working on it, or any similar technology. I do a limited amount of collaboration with other researchers interested in DAC, but in doing so, I make it clear that for DAC related work my primary responsibility is to Carbon Engineering.

As I see it, I have a clear conflict of interest if I do academic or advisory work on DAC (or related areas such as low carbon fuel policies) using my status as a professor or as an “expert” in energy and climate policy without indicating my vested interest in a company that has direct benefits from low carbon fuel policies.

That said, the concerns about my conflicts of interest have focused on the conflict between Carbon Engineering and my academic work on solar geoengineering.

Much of the concern about solar geoengineering is rooted in the fear that its development will sap efforts to cut emissions. This is often called geoengineering’s “moral hazard.” I think it’s somewhat more useful to think of it as a political risk. Put simply, I expect that work on solar geoengineering, including my own work, will be actively exploited by those who oppose emissions cuts, most obviously fossil fuel companies and fossil-rich nations. Such groups will likely exaggerate the effectiveness of solar geoengineering and minimize it’s risks in order to weaken efforts to cut emissions. Whatever it’s called, concern about political misuse of solar geoengineering research is real and serious. To my knowledge, I was the first to call it out as a “moral hazard” in a review article in 2000.

If solar geoengineering weakens climate policies, then it threatens cleantech companies like Carbon Engineering. This is not a minor issue. The only way that Carbon Engineering succeeds is with strong carbon policy. When raising funds for Carbon Engineering, one of the biggest concerns we hear from potential investors is that government policies penalizing high-carbon fuels (such as California’s Low Carbon Fuel Standard) might not be politically stable if governments waver on environmental policies.

To sum up: there would be conflict of interest if my advocacy of solar geoengineering research benefited the interests of my company. But this cannot be the case. My advocacy of solar geoengineering research is contrary to the interests of Carbon Engineering for two reasons. First: because of the potential for solar geoengineering to weaken mitigation policies, i.e. the “moral hazard”. And second, because my involvement in solar geoengineering increases the chance that Carbon Engineering will be seen as a “geoengineering” company with all the ethical and regulatory concerns that this entails.

Conversely, there would be a conflict of interest if my work at Carbon Engineering made solar geoengineering more credible. This seems implausible. Many climate policy advocates see a trade-off between solar geoengineering and carbon removal. They argue that mitigation alone cannot meet the ambitious climate targets agreed to at Paris, and either carbon removal or solar geoengineering may be needed to keep the world from warming more than two degrees. As most see carbon removal as much less risky and politically problematic than solar geoengineering, it follows that if work in Carbon Engineering and its competitors help make carbon removal more plausible, it weakens this case for solar geoengineering.

Divergent Roles for Private Capital

As explained in an earlier blog, I oppose commercial work on core solar geoengineering technologies. My essential concern is that commercial development cannot produce the level of transparency and trust the world needs to make sensible decisions about deployment. A company would have an interest in overselling, an interest in concealing risks. Solar geoengineering is not cleantech. It’s not a better battery or wind turbine. It’s a set of technologies that might allow humanity to alter the entire climate. As much as possible, it needs to be owned and controlled by transparent democratic institutions. It requires global governance.

It might be argued that, in forgoing commercial development of solar geoengineering, we lose the chance to harness commercial innovation to reduce costs. But cost is already so low that it’s more of a bug than a feature. Low cost may make it too tempting. Low cost enables unilateral action.

Why commercial work on DAC but not on solar geoengineering? It’s true that as a company, Carbon Engineering’s development process is less transparent than academic research. But transparency in the development process is not needed if the final product can be easily validated. When Carbon Engineering succeeds and large-scale air capture plants are built, it will be very easy for outside entities such as governments, third parties, or citizen groups to monitor the net flows of energy and materials in and out of the plant, as well as various industrial byproducts or emissions. The potential environmental risks of a Carbon Engineering plant are well-regulated by existing regulations on similar industries such as power plants, paper mills, and chemical plants.

This difference is linked to the fundamental difference between solar geoengineering and a carbon removal technology like DAC. Solar geoengineering is large-scale climate modification which inherently has global consequences that are difficult to quantify even after deployment. DAC results in emissions reductions (carbon-neutral synthetic fuels) or net CO_₂ removal (sequestration), with local impacts that can be measured with reasonable accuracy.

For public policy, the essential distinction between solar geoengineering and DAC rests on their very different distributions of risks, benefits, and costs: solar geoengineering entails uncertain global risks and benefits with negligible direct costs, while DAC and similar carbon removal technologies provide a global benefit in exchange for local risks and significant costs. Their very different governance challenges arise directly from this asymmetry.

Clean energy technologies like wind or nuclear power also offer the global benefit of reduced emissions in exchange for local costs and environmental risks. DAC is, as I see it, more like an energy technology than a form of geoengineering. And, the use of DAC to make carbon-neutral hydrocarbon fuels is an energy technology that competes directly with batteries and biofuels to provide low-carbon transportation. Finally, unlike solar geoengineering, there is a large public benefit to driving down the cost of DAC. That’s why I am very proud to be part of Carbon Engineering, but strongly oppose any commercial work on solar geoengineering.

Calling both “geoengineering” is misleading. Words matter.

Original post on Harvard.edu

By David Keith and John Dykema | May 3, 2018

We broadly oppose commercial development solar geoengineering. In our view, a central objective of solar geoengineering research is to develop credible assessments of its risks and efficacy. Credibility depends, in part, on confidence that the risks of solar geoengineering are not concealed, that its effectiveness is not exaggerated. Such credibility can, in our view, be best generated by a transparent multipolar research effort. Where, “transparent,” means open access to the full research process, including raw data, dead ends, and experimental failures. Where “multipolar” means the research is conducted by a diversity of independent entities including research by groups that focus on finding the ways that it will fail.

Such transparency cannot reasonably be achieved in a commercial setting that depends on the ability to protect and monetize intellectual property. We therefore disapprove of patenting of technologies that are core to the deployment or monitoring of solar geoengineering. This is not an injunction against any commercial involvement. Any research, or eventual deployment, will—of course—depend on a web of firms suppling components and services. Our concern is with the core technologies specific to solar geoengineering.

Our recent publication, Production of Sulfates Onboard an Aircraft: Implications for the Cost and Feasibility of Stratospheric Solar Geoengineering, serves as a useful example to discuss our concerns with patenting.

Unlike most of our work, this paper describes a possible improvement to technologies for solar geoengineering. It provides a chemical engineering analysis of a system to convert sulfur to SO₂ or SO₃, and could be used aboard aircraft to produce sulfate aerosols in the stratosphere for solar geoengineering. Such a system could reduce the cost and environmental impact per unit of sulfur delivered. And it could facilitate use of SO₃ to make accumulation mode H₂SO₄ particles that allow for better control of the distribution of particle sizes.

In any case, this is an example of a technology that would very likely have been patentable. However, because we oppose patenting we elected not to patent this technology. In so doing we follow the practice that we have for all solar geoengineering related research. We have never filed a patent related to solar geoengineering and have worked to find ways to block or discourage others from doing so. In 2012 for example, one of us (Keith) participated in organizing a workshop run by Granger Morgan at Carnegie Mellon University. It explored options for promoting transparency, leading to a suggestion that:

“In order to lessen the incentive for private commercial interests to influence the direction of the pursuit of SRM, it would be desirable to restrict the assertion of such private intellectual property rights to technical fields other than SRM. Federal agencies already have statutory authority to take prescribed action to restrict or partially restrict the patent rights of awardees.”

More recently, Harvard’s Solar Geoengineering Research Program was established with a policy discouraging patenting.

Our publication of the above finding contributes to transparency because it partially blocks anyone else from patenting something similar. Here’s a very rough summary of relevant patent law: Europe, and many other jurisdictions, have a so-called First To File policy with no grace period after public disclosure. This means that anyone can, in principle, file a patent whether they were the inventor or not, so long as they are the first person to file. However, once the work is publicly disclosed, it is unpatentable. The U.S. has a recently revised First Inventor To File system that includes a limited one-year grace period. The grace period means that, under new restrictions, the original authors can file a patent within one year of publication. No one else can, since the publication is prior art and other inventors are not authors of it. Note that this is a mere sketch of the issues—patent law is absurdly complicated.

In publishing this work we made it unpatentable by anyone (including us) in Europe and similar jurisdictions, and we complicated its patenting in the US. In practice, there are a lot of ways where it is likely possible for someone to patent something that treads on some of the same ground, but they would be restricted by our public disclosure of the original idea as “prior art”.

Returning to the big questions. We have mixed feelings about this paper. We are interested in improving knowledge of the risks of solar geoengineering or finding ways to reduce those risks. We have generally avoided finding ways to reduce its cost. For this publication, we chose to make an exception. To start, because low cost is potentially problematic, we thought a method that could meaningfully reduce cost without introducing significant technical complexity would be an important finding to publish. Furthermore, although this method could in principle reduce cost, its practical realization would require significant engineering and capital expenditure well beyond what is represented by our publication. Additionally, we did this work because we thought that, in this case, for sulfate aerosols, the most generic and well-studied potential method of solar geoengineering, it was worthwhile to present this idea in written form since it has already been discussed. (It was mentioned obliquely in the footnotes of a prior paper.) On balance, we felt it was better to describe the process in a technical publication. Moreover, we judged that if solar geoengineering ever moved towards deployment, a well-funded engineering effort would far surpass our effort in inventing innovative ways to reduce costs.

It might be argued that in forgoing commercial development of solar geoengineering we lose the chance to harness commercial innovation to reduce costs for core solar geoengineering technologies. But, cost is already so low that it’s more of a bug than a feature. Low costs enable unilateral action. Solar geoengineering is not a consumer product. It’s a set of technologies that might allow humanity to alter the climate over decades to centuries. As much as possible, it needs to be owned and controlled by transparent democratic institutions. It requires global governance.

Original post on Harvard.edu